The invention is related to the field of electromagnetic well logging instruments and methods. More specifically, the invention is related to an apparatus and method for improving the performance of nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) well logging instruments by increasing the strength of the permanent magnet.
Electromagnetic well logging instruments include circuits connected to antennas which induce alternating electromagnetic fields in earth formations surrounding a wellbores, and include circuits which measure various electromagnetic phenomena which occur as a result of interaction of the alternating electromagnetic fields with the earth formations. Such electromagnetic phenomena relate to petrophysical properties of interest of the earth formations. One type of electromagnetic well logging instrument which suffers deleterious effects of eddy currents in electrically conductive elements of the logging instrument is the nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) instrument. One type of NMR instrument is described in U.S. Pat. No. 4,710,713 to Taicher et al. Another type of NMR instrument is described in U.S. Pat. No. 4,350,955 to Jackson et al. Both the Taicher ""713 instrument and the Jackson ""955 instrument include permanent magnets for inducing a static magnetic field in earth formations, and an antenna through which pulses of radio frequency (xe2x80x9cRFxe2x80x9d) energy are conducted. RF energy conducted through the antenna induces an RF magnetic field in the wellbore, in any electrically conductive elements of the NMR instrument and in the earth formations surrounding the instrument. The RF energy passing through the antenna of the NMR instrument therefore causes eddy currents to flow in the wellbore, in the earth formation surrounding the NMR instrument and in any electrically conductive elements in the NMR tool.
In the Jackson ""955 patent the antenna acts as a three-dimensional dipole. The direction of a magnetic field generated by the antenna is generally along the direction of the dipole and parallel to its longitudinal axis. This type of antenna is generally referred to as a longitudinal dipole. The antenna induces an RF magnetic field in the wellbore, in the earth formations surrounding the tool and in the permanent magnet material on both sides of the dipole along the longitudinal axis of the tool. To induce an RF magnetic field in the earth formations having sufficient amplitude to make useful NMR measurements, the antenna must also necessarily generate a relatively strong RF magnetic field within the permanent magnet. If the permanent magnet material is electrically conductive, losses of RF power will occur as a result.
The apparatus disclosed in the Taicher ""713 patent includes a substantially cylindrical permanent magnet assembly which is magnetized perpendicular to its longitudinal axis. This magnet can be modeled as an infinitely long two-dimensional dipole. The magnet induces a static magnetic field in the wellbore and in the earth formations which has substantially uniform magnetic field strength within any thin annular cylindrical volume at a predetermined radial distance from the magnet. The Taicher ""713 apparatus also includes an antenna, wound around the exterior of the magnet, for generating the RF magnetic field and for receiving NMR signals. This antenna can be modeled as an infinitely long two-dimensional dipole. The direction of the magnetic field generated by this antenna is generally perpendicular to its longitudinal axis. This type of antenna is referred to as a transversal dipole antenna. The permanent magnet""s dipole is coaxial with and orthogonal to the RF magnetic dipole.
The apparatus disclosed in the Taicher ""713 patent has several drawbacks. In particular, the antenna induces an RF magnetic field in the formations surrounding the tool which decreases in strength as the square of the radial distance from the magnet axis. Therefore, to induce an RF magnetic field in the earth formations having sufficient amplitude to make useful NMR measurements within a sensitive volume in the earth formations, the antenna must generate a very strong RF magnetic field, which is also very strong within the space that is occupied by the permanent magnet. If the magnet is made from electrically conductive permanent magnet material, significant losses of RF power will occur as a result of eddy currents flowing in the magnet. The apparatus disclosed in the Taicher ""713 patent is generally useful only with an electrically non-conducting permanent magnet material such as ferrite.
Choosing ferrite magnets has a number of consequences. First, the tool diameter and the weight are large because much more magnet material is needed to satisfy the performance criteria. The required diameter reduces the number of wells that can be logged with a tool using ferrite magnets, and smaller diameter tools have decreased performance due to the weaker static field, depth of investigation and signal to noise ratio of the NMR signals. The most significant effect of using ferrite magnets is to induce a large amount of magnetoacoustic ringing in the NMR signals: acoustic waves induced by the RF in the magnet couple back into the antenna creating a signal that may be much larger than the desired NMR signal.
Another NMR logging instrument is described in U.S. Pat. No. 5,055,787 to Kleinberg et al. This logging instrument includes permanent magnets arranged to induce a magnetic field in the earth formation having substantially zero field gradient within a predetermined sensitive volume. The magnets are arranged in a portion of the tool housing which is typically placed in contact with the wall of the wellbore. The antenna in the instrument described in the Kleinberg ""787 patent is positioned in a recess located external to the tool housing, enabling the tool housing to be constructed of high strength material such as steel. This outside metallic structure also serves as a shield against RF alternating electromagnetic fields penetrating into the permanent magnet and resulting in RF power losses in the magnet.
Although instrument in the Kleinberg ""787 patent reduces eddy current losses in electrically conductive elements of the tool by shielding the permanent magnet, this concept has several significant drawbacks. One such drawback is that the instrument""s sensitive volume is only about 0.8 cm away from the tool surface and extends only to about 2.5 cm radially outward from the tool surface. Measurements made by this instrument tool are therefore subject to large error caused by roughness in the wall of the wellbore, deposits of the solid phase of the drilling mud (called xe2x80x9cmudcakexe2x80x9d) onto the wall of the wellbore in any substantial thickness, and by the fluid content of the formation in the invaded zone.
Another way to reduce eddy current losses in the permanent magnet in an NMR logging apparatus is described in U.S. Pat. Nos. 5,376,884 and 5,486,761 to Sezginer. The instruments described in these patents use side-by-side spaced apart elongated magnets and an RF loop in the region between the magnets. Such an arrangement enables using relatively powerful permanent magnets, such as rare-earth magnets, provided that the permanent magnets are properly shielded. The basic disadvantage of the approach taken in the Sezginer ""884 and ""761 patents is that the relatively large conducting surfaces will disturb the spatial distribution of the RF magnetic field while transmitting, and will reduce the signal to noise ratio (xe2x80x9cS/Nxe2x80x9d) while receiving NMR signals.
Generally speaking, the measurement approaches suggested in the Jackson ""950 and the Taicher ""713 patents are commercially preferred for making NMR measurements of earth formations. The apparatus described in both of these patents are preferably used with substantially non-conductive permanent magnets. Magnetic materials used to make permanent magnets generally fall into two classes: ferrites, which are oxides of ferromagnetic metals; and ferromagnetic metals and their alloys combined with other metals and/or rare earth elements. The first class generally consists of non-conductive permanent magnet materials, and the second class generally consists of electrically conductive materials. Both classes of permanent magnet materials can be used in making so-called xe2x80x9cbondedxe2x80x9d permanent magnets. Bonded permanent magnets are generally manufactured by pressure bonding or injection molding of magnet material powders in a carrier matrix. The carrier matrix is typically formed from an electrically non-conductive polymeric or epoxy resin. The magnet material density of this form of magnets is lower than magnets made entirely from sintered metallic materials, yielding lower magnetic strength properties in the final product.
U.S. Pat. No. 6,069,479 to Taicher et al, the contents of which are fully incorporated herein by reference, discloses the use of a magnet formed from a powdered, electrically conductive permanent magnet material. The grain size of the magnet material small enough with respect to the frequency of the radio frequency magnetic field to substantially prevent intragranular power loss of the radio frequency magnetic field. The magnet is assembled from electrically isolated blocks of the magnet material each having a thickness less than a skin depth of the radio frequency magnetic field within the magnet material. The boundaries between contiguous ones of the blocks are oriented to interrupt flow of eddy currents in the magnet resulting from the radio frequency magnetic field. In a preferred embodiment of the invention, the magnet material comprises powdered, epoxy resin-bonded Samarium-Cobalt.
Two features limit the effectiveness of the Taicher ""479 device. First, the grains of Samarium-Cobalt must satisfy a size constraint dictated by the operating frequency of the tool. The particle diameter must be less than the skin depth of the radio frequency (RF) at the maximum operating frequency. Secondly, the magnet in the Taicher ""479 device must be segmented into electrically isolated blocks. This requirement arises from the fact that the epoxy resin-bonded Samarium-Cobalt material has a significant residual conductivity.
The segmentation needed in the Taicher ""497 makes the construction of the magnet more complicated: it would be desirable to have a magnet with high remnant magnetization and coercive force without having associated high eddy current losses. It would be desirable to have a permanent magnet material that has lower magnetoacoustic ringing than ferrites. The present invention satisfies the need.
The present invention is an NMR apparatus having a bonded non-conducting magnet that includes grains of a conducting magnet material at a volume fraction close to a percolation limit wherein the grains form a continuous conducting path through the magnet. The bonding material is preferably epoxy resin while the conducting material may be Samarium-Cobalt or Neodymium-Iron-Boron. In a preferred embodiment of the invention, the magnet includes grains of a ferrite interspersed between the grains of the conductive material, the ferrite grains being smaller than the grains of conductive material. Optionally, an inert material may be used instead of a ferrite as a filler. The grain size may be slightly smaller than the skin depth of the RF field at which the NMR apparatus operates.